One type of drill bit used in forming a borehole in the earth is a roller cone rock bit. A typical roller cone rock bit comprises a body with an upper end adapted for connection to a drill string. A plurality of arms, typically three, depend from the lower end portion of the body with a cutter cone mounted on each arm. Each arm generally includes a spindle protruding radially inward and downward with respect to a projected rotational axis of the body. Each cutter cone also has an internal cavity shaped to receive an associated spindle. Each cutter cone may be mounted on a respective spindle and rotatably supported on bearings acting between the spindle and the inside of the cavity in the respective cutter cone. One or more nozzles are often located on the underside of the body and radially inward of the arms. These nozzles .generally are positioned to direct drilling fluid passing downwardly from the drill string to the bottom of the :borehole being formed. The drilling fluid washes away the material removed from the bottom of the borehole and cleanses the cutter cones carrying the cuttings radially outward then upward within the annulus defined between the bit body and the wall of the borehole.
Protection of the bearings that allow rotation of the cutter cones can lengthen the useful service life of a roller cone rock bit. Once drilling debris or external fluids are allowed to infiltrate between the bearing surfaces of the respective cutter cone and the spindle, failure of the roller cone rock bit will follow shortly. Various mechanisms are employed to keep debris and external fluids from entering between the bearing surfaces. A typical approach is to utilize an elastomer seal across the gap between the bearing surfaces of the rotating cutter cone and its associated spindle. However, when this seal fails, it is not long before external fluids or drilling debris will flow through the gap between the cutter cone and the spindle and contaminate the bearing surfaces. Thus, it is important that the seal be fully protected against wear caused by debris in the borehole and protected from pressure exerted by external fluids, such as drilling fluid or water, present in the borehole.
In a sealed roller cone rock bit, the cavity in the cutter cone and the bearings are often lubricated by packing the cavity and bearings with a lubricant such as grease. This lubricant is preferably sealed from the external environment by the elastomer seal across the gap between the cutter cone and the spindle. It is important for satisfactory bearing performance that the lubricant not be contaminated with external fluids or debris. This seal is especially important when the roller cone rock bit is used in drilling a deep well, such as a deep oil or gas well, where the borehole is filled with a column of water or other fluids exerting tremendous pressure on the seal. In such a high pressure downhole environment, the external fluids exert constant pressure on the outer side of the seal. If the seal fails, external fluids can mix with the lubricant and quickly cause failure of the roller cone rock bit. Thus, it is important to maintain the integrity of the seal.
One prior sealing system utilizes a sealed pressure equalizing lubrication system such that the pressure of the lubricant and the pressure of the external fluids are equalized through the operation of a diaphragm. An example of this system is shown in U.S. Pat. No. 4,597,455. The use of the diaphragm insures that the pressure on both sides of the seal is substantially equal. By equalizing the pressure, the lubrication system helps to protect the seal preventing debris, water and other external fluids from contaminating the lubrication system. In particular, the system disclosed in U.S. Pat. No. 4,597,455 places a pressure equalizing lubricant reservoir in the ball passage used to load ball bearings between the cutter cone and the spindle.
An additional safe guard used in some roller cone rock bits is a second seal outward from the first seal between each cutter cone and its associated spindle. The addition of a second seal creates a seal-gap between the second seal and the first seal. The second seal acts as an initial barrier to the external fluids and debris. An example of a roller cone rock bit having a seal-gap is described in U.S. Pat. No. 5,027,911 which shows a seal-gap that is connected to the bearings to allow lubricant to migrate from the bearings to the seal-gap.
It is important that the outer, second seal functions to protect the inner, first seal from external fluids and debris. In conventional roller cone rock bits, the second seal and the first seal cannot both be fluid tight seals, such as elastomer ring seals. If both seals were fluid-tight, a pressure differential may occur in the seal-gap between the first and second fluid tight seals. A difference in the pressure of the external fluids and the pressure of the lubricant as compared to the pressure in the seal gap would force each fluid-tight elastomer seal into the seal-gap or pull them away from the seal-gap. This pressure difference causes extrusion of the elastomer seals and cutting of the respective elastomer as the cutter cone rotates. Once an elastomer seal is cut, the integrity of the seal is lost, and the roller cone rock bit will fail shortly. The fact that the second elastomer seal cannot be fluid-tight in conventional roller cone bits limits the effectiveness of the second seal.